1. Field of the Invention
The present invention relates to generation of transmission signal frame with respect to orthogonal frequency division multiplexing (hereinafter, referred to as “OFDM”) transmission system; and more particularly to an OFDM signal frame generator, a transmitter, a signal transmission system and an OFDM signal frame generating method.
2. Related Background Art
Recently, an OFDM transmission system has been applied to a radio LAN system such as IEEE.11a for practical use, and it is also under examination to apply the OFDM transmission system to surface digital broadcasting and cellar communication. The OFDM signal transmission system does not require frequency response equalization of multipath interference, which is caused from multiple propagation path, and then the system is suitable for broadband signal transmission.
Generally, in radio communication, the conditions of propagation path vary depending on the location of a reception terminal. Accordingly, adaptive modulation/demodulation technique, which changes the transmission rate at the transmitting party in accordance with the conditions of the propagation path, is employed. When carrying out the adaptive modulation/demodulation, a transmitter obtains value of received signal quality from a reception party, and changes the transmission rate in accordance with the value of received signal quality. The value of received signal quality is determined at the reception party using a desired value of received signal quality (desired frame error ratio), received signal power value, received signal noise-power ratio and Doppler frequency.
FIG. 1 shows an example of the conventional OFDM signal frame generator 93, to which the adaptive modulation/demodulation technique is applied. A transmission section comprises a frame generator 93, an OFDM modulator 92 and an RF section 91.
In the frame generator 93, the rate information determiner 94 determines the rate information using the value of received signal quality. Needless to say about the number of information bits, the encoding rate, the modulation order and the spreading factor, the rate information may include information other that the above or only a part of information of the above. A data symbol series generator 95 generates a piece of data symbol series in accordance with the rate information. When the reception party requires the rate information for demodulating, the rate information is also included in the information symbol series. A pilot symbol generator 96 generates pilot symbols. A multiplexer 97 arranges the generated data symbols and pilot symbols to a time-frequency slot, and generates a transmission signal frame. An OFDM modulator 92 OFDM-modulates the signal in the frame. The OFDM modulated signal is subjected to frequency conversion and transmitted by the RF section 91.
FIG. 2 shows a block diagram of an OFDM transceiver employing the conventional frame generator. A data symbol generator 95 in a transmitter 90 comprises an encoder 95A, mapping unit 95B for generating modulation order and a spreading unit 95C.
On the other hand, a receiver 80 comprises an RF section 81, an OFDM demodulator 82, a channel estimator 83, a received signal quality determiner 85 and a despreading/demapping/decoding unit 84. The received signal is downconverted by an RF section 81 and subjected to OFDM demodulation by an OFDM demodulator 82. The channel estimator 83 carries out channel estimation using the OFDM-demodulated signal and the pilot symbols. The channel estimator 83 estimates the channel value (amplitude and phase) of every sub-carrier used for transmission and the value of power of noise applied to a reception antenna. The received signal quality determiner 85 calculates the received signal-noise power ratio using the estimated channel value and the estimated noise power value. A value of received signal quality is determined using these values and a desired value of received signal quality. The despreading/demapping/decoding unit 84 demodulates the information symbol series using the estimated channel value and the OFDM-demodulated signal.
FIG. 3 shows an example of a generated transmission frame. Detailed OFDM signal parameters are shown in FIG. 12. The pilot & data arrangement format has such structure that, for each sub-carrier, 4 pilot symbols are arranged respectively at the head and tail of the frame, and 12 data symbols are arranged in the center of the frame. FIG. 13 shows a corresponding table of the value of received signal quality, the rate information (encoding rate, modulation order, spreading factor, number of information bit) and the resulting transmission rate, in the case where the pilot & data arrangement format in FIG. 3 is used. The rate determiner determines the above-mentioned rate information using the information in the corresponding table in FIG. 13 corresponding to the value of received signal quality “1-3”, which is fed back from the receiver 80. For example, when the received signal quality is “1”, to an input information symbol series of “945 bits”, 12 bits of CRC (Cyclic Redundancy Check) bits are added resulting in 957 bits (approximately 960 bits). Furthermore, by carrying out FEC (Forward Error Correction) with encoding rate “½” and QPSK (Quadrature Phase Shift Keying) with 2 bits/Hz to spread with spreading factor “1”, data symbol of 960 symbols are generated as symbols after encoding/modulating/spreading processing.
In the demodulation of the OFDM signal transmission, it is necessary to estimate the channel value of every sub-carrier at the reception party. Here, it is assumed that, as a channel estimation method, which is simple and has high estimation accuracy, in each sub-carrier, total eight pilot symbols are subjected to a synchronous addition to estimate the channel (it is assumed that the channel variation in a frame is extremely small).
In FIG. 3, Pp/Pi indicates pilot symbol power (Pp) per sub-carrier and power ratio per modulated symbol after despreading. Generally, to obtain satisfactory channel estimation accuracy, the value (Pp/Pi) is preferably about 6-10 dB. In the case of FIG. 3, Pp/Pi is about 9 dB. In the transmission signal frame, which is generated by the above conventional frame generator, the pilot & data arrangement format and the number of pilot symbols are fixed irrespective of the value of received signal quality.
A technique, in which the number of pilot symbols is adaptively changed to reliably carry out the OFDM communication, has been proposed (refer to Japanese Patent Application Laid-open Gazette No. 2000-151548).